The increasing cost of fossil fuels has generated a strong interest in renewable solar energy sources. Although relatively efficient, inorganic photovoltaic devices can be costly. Advances in organic bulk heterojunction (“BHJ”) photovoltaic devices have opened prospects for the development of less expensive, easily produced alternatives. To date, solar conversion efficiencies of organic BHJ devices have reached about 5-6 percent, still short of the desired 10 percent for commercialization and widespread adoption.
The more efficient organic BHJ devices have been based on a conjugated semiconducting polymer blended or mixed with a fullerene derivative. The fullerene derivative serves as an electron acceptor material to split excitons formed when the polymer absorbs light. The polymer and fullerene components should be mixed together on a length scale that is shorter than an exciton diffusion length, typically about 10 nm, to ensure that most excitons are harvested. It is also desirable that the polymer and fullerene components form bicontinuous interpenetrating networks for efficient transport of separated charge carriers to respective electrodes. Forming an optimal BHJ device is a difficult balancing act: having the components mixed too well can impede the formation of separate polymer and fullerene networks, but too much component separation can lead to the formation of unconnected islands that serve as charge carrier traps. In view of the difficulty of achieving this balance, low performance in BHJ devices remain a significant obstacle.
It is against this background that a need arose to develop the photovoltaic devices and related systems and methods described herein.